Focusing device in a multi-cavity klystron



W. SCHMIDT July 21, 1970 FOCUSING DEVICE IN 'A MULTI-CAVITY KLYSTRON Filed May 115, 1968 INVENTOR WOLFGANG SCHMIDT AGENT United States Patent Int. Cl. Hin 23/18, 23/20 U.S. Cl. SIS-5.34 4 Claims ABSTRACT OF THE DISCLOSURE A multi-cavity klystron in which the cavities are connected by drift spaces with those adjacent the electron gun being electrostatically focused while those adjoining the collector are magnetically focused.

The invention relates to a focusing device for guiding an electron beam in a multi-cavity klystron having at least two electrostatic focusings of the electron beam in the drift space adjoining an anode of an electron gun.

Electrostatic focusing of the electron beam in a onecavity klystron is known from the U8. patent specification 2,567,674. In this one-cavity klystron the electron beam which emerges from a cathode having a large surface area is focused in the electron gun by an electrostatic focusing onto an exit aperture of the electron gun which has a substantially smaller diameter than the cathode of the electron gun. Such an electrostatic focusing may be effected either by the particular shape of the anode, for example, by constructing the anode as an exponential horn, or by constructing the anode as several bodies, which like in the case of a lens, are arranged behind each other in the direction of the electron beam. In this known arrangement the so-called inter-action space in which a coupling loop is arranged for coupling out the power at the end of the electron gun, so immediately adjoining the anode. The known arrangement shows the use of a permanent magnet the magnetic field of which extends beyond said interaction sapce and is provided for a better guiding of the electrons.

However, for multi-cavity klystrons, so far only either an electromagnetic or permanent magnetic or an electrostatic focusing has become known. The advantage of the electrostatic focusing is the simple construction of the multi-cavity klystron and the complete failure of heavy and comparatively space-consuming magnet yoke portions, so portions of soft iron, which are to be provided for guiding the magnetic field. The drawback of the exclusively electrostatic focusing consists in that in the case of strong beam modulation in the last drift space, the electrons, which are retarded in their velocity are further retarded in the negative potential fields of the focusing electrodes or are even reflected, particularly at the edge regions. The slow electrons are selected from the beam and attracted by the drift electrodes as loss current. They are lost for the inter-action in the coupling-out gap may also produce disturbing oscillation phenomena.

In order to avoid these drawbacks the invention proposes, for a better guiding of the electron beam, a focusing device of the type mentioned in the preamble which is characterized in that in the drift space an electromagnetic or permanent magnetic focusing unit, which extends 3,521,117 Patented July 21, 1970 over at least the last two drift spaces and the exit gap, adjoins the electrostatic focusing. As a result of this, the electron beam in the exit gap is guided with certainty and also a better introduction of the electron beam in the collector is effected.

According to the invention, for a better guiding of the magnetic field, soft iron components may be used in the exit gap in constructing the exit cavity. A cavity preceding the exit cavity may also contain soft iron components. A collector may comprise a sealing ring which consists of soft iron on its end face facing the exit gap.

One example is shown in the drawing and will be described in detail below.

The drawing shows a multi-cavity klystron having four cavities. The essential parts of said multi-cavity klystron are an electron gun 1 consisting inter alia of a cathode 2 and an anode 3 for concentrating the electron beam denoted by the broken lines 4. At the end of the electron gun 1 a drift space 5 is provided. The drift space begins behind the anode 3. On the other side of the multi-cavity klystron, a collector 6 is arranged, in the cavity 25 of which the electron beam 4 is caught. Between the electron gun I and the collector 6 of the multi-cavity klystron, several cavities 7 are arranged which consist of non-evacuated cavity portions 9 and evacuated cavity portions 8. The evacuated cavity portions are constructed in known manner from two cavity flanges 10 and 11 between which an insulating seramic ring 12 is arranged. In the non-evacuated cavity portions 9 the short circuit slides are arranged for tuning which are not shown to avoid complexity of the drawing. The interaction spaces 13, 14, 15 and 16 are provided in the evacuated cavity portions, the last of which (16) is also referred to as a so-called exit gap. Between these individual interaction spaces, the cavities 7 are arranged which connect the drift spaces 17, 18 and 19, and behind the exit gap 16 there is the drift space 20 to which the collector 6 of the multi-cavity klystron adjoins. The drift spaces 5, 17 and 18 are interrupted in known manner by electrostatic focusing systems which produce a sharp concentration of the electron beam to the next interaction gap. The electrostatic focusing units are at a common negative voltage 21 which is equal to the cathode potential.

For better guiding the electron beam 4 in the exit gap 16, according to the invention electromagnetic or permanent magnetic systems 22 and 23 are provided at the area of the drift space 19 and the drift space 20 and produce a magnetic field the field lines of which are shown as dot-and-dash lines. The magnetic field is concentrated substantially over the exit gap but may also scatter over the interaction gap 15. For a better guiding of the magnetic field, the cavity flanges 11 and 12 of the two last evacuated klystron cavity portions 8 may consist of soft iron. The non-evacuated cavity portions 9 of the last two cavities may also contain soft iron portions on their outer circumference as denoted by 24. Finally, in order to introduce the electron beam better into the collector cavity 25, the end face of the collector is provided with a sealing ring 26 which consists of soft iron.

Pole-shoes may adjoin in known manner the cavity flanges 11 and 12 on the drift space and enable a particular guiding of the magnetic field.

What is claimed is:

1. A multi-cavity klystron comprising an electron beam source and a collector defining an electron beam path, a

plurality of resonant cavities connected by drift spaces in the path of the electron beam and an exit gap between the collector and the last cavity, means to electrostatically focus the electron beam in at least two successive drift spaces following the beam source, and means to mag- 5 netically focus the beam in the exit gap and at least one of the preceding drift spaces in which the beam is not electrostatically focused adjoining drift spaces in which the beam is electrostatically focused.

2. A multi-cavity klystron as claimed in claim 1 in which the exit gap is bounded by soft iron parts to improve the magnetic field in the cavity.

3. A multi-cavity klystron as claimed in claim 2 in which the walls of the next preceding cavity include soft iron portions for further improving focusing of the beam. 15

9/1966 El-Hefni SIS-5.52 X 7/1968 Schmidt 3155.35

10 ELI LIEBERMAN, Primary Examiner S. CHATMON, JR., Assistant Examiner US. Cl. X.R.

ff UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3521117 D t d July 21, 1970 Inventor (s) WOLFGANG SCHMIDT It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 8, change "May 15, 1967" to -May 18, l967;

Column 1, line 9, change "1,541,961" to SKSNED AM 321110 NOV. 17,1970

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